We derive an explanation of the third class of sympathetic phenomena from the known laws of sensitive and motor nerves. It is known that stimulation of a sensitive nerve at its origin, or in any part of its course, will give rise to a sensation which will be re ferred to the peripheral extremity of the sti mulated fibres ; and that a stimulus applied to a motor nerve causes a change in it which spreads peripherad from the point stimulated, and therefore affects the muscular parts with which it is connected. It is known also, that a sentient nerve may excite a motor or sensi tive nerve which is implanted near to it in the nervous centre — doubtless through the change which it produces in that centre ; nor can it be doubted that a sensitive nerve may receive such a powerful stimulus as to exalt the polar force of a large portion of the nervous centre in the neighbourhood of its insertion, and thus to excite a similar change in all the nerves, whether motor or sensitive, which are connected with it. Thus, ac cording to the intensity of the original stimu lus, there will be a radiation of nervous force from the centre, either in one or two motor or sensitive nerves, or in several such ; and the number and variety of the sympathetic phenomena will thus depend on the intensity and extent of the change in the nervous centre excited by the primary stimulus.
To explain, then, the phenomena of sensa. tion and motion under consideration, we must determine the individual nerves affbcted in each instance, and ascertain what con nexions they have with each other. We learn from anatomical investigation, that, al though nerves anastomose with each other in their distribution, this anastomosis is by no means of that kind which would justify the supposition that an irritation could be com municated from one to the other in their course. The nerve-fibres only lie in juxta position, but do not communicate ; and there is an evident provision in the tubular mem brane and white substance of Schwann for the insulation of the central axis, which is probably the effective substance in the nervous action. We must seek, therefore, in the nervous centres for such a communication be tween these nerves as may eitplain the excita bility of one by the other. In the present state of our knowledge we can do no more than state it as in the highest degree pro bable that nerves implanted in the centre im mediately contiguous to each other can exert an influence upon the vesicular matter of the centre, and upon each other.
But there are certain facts which strate beyond all doubt, that, in such actions ' as we refer to, the integrity of the centre ' forms a necessary condition. First, in many of the instances .it is plain that there can be , no connexion between the affected nerves elsewhere than in the centre, for they are so distinct from each other that there is not even that apparent connexion which results from the anastomosis of a fasciculus of fibres of the one with a portion of the other. Secondly, the removal of the portion of the nervous centre with which any one of the nerves concerned in the sympathetic action is connected, will prevent the development of the phenomenon, although the nerves themselves remain un injured in their peripheral distribution, or in their connexion with each other. Thirdly, if
there were any peripheral communication be tween nerves, it would be most likely to take place in the plexuses. Experiments, how ever, upon the nerves which lead to these show that each nerve-tube, in its passage through them, retains its isolation as dis tinctly as in any other part of its course. The three nerves which supply the lower extremity in the frog, says MUller, form a plexus from which two nervous trunks issue: if one of these latter be divided and isolated from all its connexions with muscles, and the portion of it connected with the plexus irri tated, the impression will be transmitted in the centripetal direction by the sensitive fibres of nerve; but the motor fibres of the other nerve arising from the plexus are not affected, and excite no contractions in the muscles to which they are distributed.* In applying these principles to the expla nation of the instances which we have quoted, we shall find it difficult to determine the cen tral connexion in some, although in others such a connexion is highly probable. It re mains, therefore, for future anatomical re search to ascertain what that connexion is which enables one nerve to sympathise with another. In the instance of pain in the shoulder in sympathy with irritation of the liver, the hepatic irritation excites a change in some sensitive nerves, which is propagated to the centre, and there affects some of the sentient fibres distributed in the region of the shoulder. The phrenic and the external tho racic nerves are both or either of them, bat more especially the former, favourably situ ated to constitute the excitant of such a sympathetic sensation. The phrenic nerve of the right side is largely distributed upon the peritoneal. surface of the diaphragm, and upon the inferior vena cave, and forms many connexions with the hepatic plexus in the substance of the liver. It may therefore readily participate in any irritation of that organ. Now, the phrenic nerve is implanted in the spinal cord on a level with the third or fourth cervical nerves ; and the nerves of the shoulder form their connexion with this cen tral organ about the same level. The origins of these nerves, therefore, are sufficiently contiguous to each other to warrant the be lief that an irritated state of one may be pro-. pagated to the other through the vesicular matter of the centre. But it may be inquired. why the irritation is limited to sensitive nerves of the shoulder ; and why movements are not excited by the stimulation of the motor fibres of the phrenic itself, or of other nerves. The limitation of the irritation to one or two nerves depends on the degree of the stimulus, and the absence of any move ments is due to the disposition of the phrenic nerve on the surface being unfavourable for the excitation of motions by irritation of its peripheral branches. And the experiment cited from Miiller, in the last paragraph, shows that simple irritation of the trunk of a compound nerve in connexion with the centre is not sufficient to produce motion ; which requires probably either a more prolonged and violent irritation of the nerve, or a polar state of the centre in which it is implanted.